Collars or necks are required on workpieces, such as billets or other finished parts, for various purposes, for example as guide supports, stiffening supports, or thread supports. The collar required for this is for example produced by means of a forming method on the workpiece, so called collar drawing or necking.
Whenever an exact form of the collar is important, such as its length or its diameter, first a punching of the workpiece may be performed, for example. Next, using a partly conical, pointed or rounded collar stamp, the previously created hole can be widened, generally forming a collar perpendicular to the workpiece. The degree of the widening can be made dependent on the collar length to be achieved and/or the size of the widening to be attained.
In the event that the collar length and/or the edges of the collar need not meet any particular quality demands, it is known alternatively to form the collar without previous punching of the workpiece, for example by means of flowdrilling or pressing. In the event of flowdrilling, it is possible to increase the length of the collar on account of a local bulk forming and the thus increased material reserve and an incremental forming, but at times the dimensional stability and precision of the necking suffers in this way.
However, when producing the collar without local bulk forming it should be kept in mind that the widening of material constitutes a substantial strain on the particular material, since the edge regions in particular are extremely stretched. If the stretching ability of the material is exceeded in this process, locally either limit strain will occur with reduced application possibilities or the material will fail, cracking or breaking open in the region of the collar.
Thus, the hole widening capability is dependent not only on the initial dimension of the hole or the dimension to be attained, but also on the material itself, which sets limits on the achievable collar size and/or collar length. Thus, for example, soft deep-drawing steels have a good hole widening capability. Accordingly, the hole widening capability is less in the case of high-strength steels, so that in consequence the collar length achievable with these materials may be greatly limited in particular.
Yet in order to increase nevertheless the achievable collar length, it is known for example from the document U.S. Pat. No. 1,613,961, in order to avoid cracks during the forming of necks, to first form a hole or opening in a flat metal sheet. Next, the sheet is placed in a die and the opening of the flat punched sheet is widened by means of a conical stamp and a collar is formed. The material around the hole should be sufficient to form the collar with the desired length.
From document DE 10 2006 029 124 A1 there is likewise known a method and a device for necking of sheet metal by using a stamp and an anvil, in which the forming height of the necked collar should be obtained in a secure process. A flat metal sheet provided with a hole is placed in a device and a collar is formed there by means of a stamp and a die. Improved process safety is supposed to be achieved in that an anvil has a hinged design, in order to better lie against the metal sheet.
However, the approaches thus far for the forming of collars, especially in terms of their length, are either not satisfactory enough or the implementation is relatively costly, for example, by the implementation of hydroforming. Therefore, there continues to be a need to make possible increased collar lengths, even for high-strength and other metals or high-strength steels, but without having to resort to rather costly methods such as hydroforming.
Especially in the case of an unfavorable drawing condition, such as when the original workpiece diameter or the flange region is large in comparison to the diameter of the deep-drawn region, there continues to be a need to enlarge the attainable collar length.
Thus, a need exists for a method and a device for producing a collar on a workpiece, wherein great collar lengths with good quality can be achieved in simple manner, even when using high-strength metals, in particular high-strength steels.